The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to reducing the x-ray dose applied to a patient without significantly increasing noise artifacts in the image.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display. In a typical CT scan, data is acquired from which a set of 2D images taken through contiguous slices may be reconstructed. In such scans, the patient table may be moved after each revolution of the gantry to acquire attenuation data for the next slice. In the alternative, a so-called helical scan may be performed in which the table is slowly moved as the gantry is revolved to acquire all the slices in one continuous motion.
Quantum noise degrades the diagnostic quality of a CT image and this noise is related to the amount of x-rays, or "dose", employed to acquire the attenuation measurements, and to the attenuation characteristics of the patient. Image artifacts due to noise will increase if the x-rays measured at the detectors drop to low levels either because the prescribed x-ray dose is too low or the beam is highly attenuated by patient anatomy. The x-ray dose is controlled by the filament current ("mA") applied to the x-ray tube, and the practice is to fix this current at a level which provides a constant dose during the entire scan. If the operator prescribes a high dose, image quality is superb throughout, but excessive x-ray flux is produced during portions of the scan when patient attenuation is low. The patient is thus exposed to an excessive dose and the x-ray tube is unnecessarily heated. On the other hand, if the dose is reduced (to prevent tube overheating during the prescribed scan), noise artifacts will appear in the image at locations where the beam is highly attenuated, in the hips or shoulders, for example. Technologists try to deal with these variables to optimize the prescribed dose by visually estimating parameters for a given patient.